Impedance matching device having a folded tapered line



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AGENT tat IMPEDANCE MATC i DEVICE HAVING A FOLDED TAPERED LINE Filed Nov. 3, 1958, Ser. No. 771,300

1 Claim. (Cl. 33334) This invention pertains to high-frequency, impedance matching line couplers and particularly to transformers having folded tapered lines and coaxial impedance coupling sections for matching over a wide frequency range.

Tapered lines that have a gradual change in spacing along their length to change their reactance as a function of the distance along the line are useful in connecting a line having one characteristic impedance to a line having a different characteristic impedance. Although a matching line may have different tapers for different applications, an exponential taper which may be calculated readily, is often used. A line having an exponential taper is described in the article Transmission Lines With Exponential Taper by Harold A. Wheeler, published in the January 1939 issue of the Proceedings of the Institute of Radio Engineers. Since the limit of the low-frequency response of the matching transmission line is dependent upon a low rate of taper, a space 100 feet or more in length is often required in order to obtain a desired impedance match. Furthermore, variations in dimensions from those used as a basis for mathematical calculations may cause the impedance characteristics of the line to depart sufiiciently from the expected calculated results that'time consuming empirical methods must be employed in order to find the required taper for obtaining desired impedance transforming characteristics.

The present invention comprises a transformer that includes a tapered transmission line that is folded so that it can be contained within a box of such dimensions that it can be readily transported. The transformer also includes a coaxial type matching section that is connected to low-impedance line terminals of the folded transmission line. The addition of the coaxial matching section increases the impedance ratio between the external line terminals of the transformer and extends the frequency range over which the transformer will operate effectively.

An object of the present invention is to confine a long tapered transmission line to a box that is small enough to be transported readily.

Another object is to provide an unbalanced to balanced coaxial type impedance matching section in combination with the line to extend the frequency range of the combination and to increase the ratio of the impedance between the incoming-outgoing line terminals of the combination.

The objects and features of the invention may be more readily understood by reading the following description with reference to the accompanying drawings in which:

Figure 1 is a top oblique view of the transformer having portions cut away to show a folded, tapered transmission line and a coaxial impedance matching section;

Figure 2 is a sectional top view of a portion of the transformer to show placement of one of the conductors of the tapered line;

Figure 3 isa cross-sectional end view of a portion of the tapered transmission line;

Figure 4 is a detailed drawing of the coaxial impedance matching section of the transformer; and

Patented Feb. 28, 1961 Figure 5 is a schematic diagram of the impedance matching section in combination with the folded tapered transmission line.

The transformer shown in Figure 1 comprises a folded tapered transmission line 10 having a pair of conductors 11 and 12 and a coaxial impedance matching section 13 having two coaxial transmission line sections 14 and 15. External connector 16 provides a connecting terminal for an external low-impedance unbalanced line; for example, a SO-ohm coaxial transmission line. As described subsequently the coaxial sections 14 and 15 that are connected between connector 16 and low-impedance line terminals 17 and 18 provide an impedance ratio of 4:1 so that if the unbalanced input is 50 ohms, the impedance of a balanced line connected to terminals 17 and 18 should be 200 ohms in order to provide proper matching characteristics. The folded tapered transmission line 10 is connected from low-impedance line terminals 17, 18 to high-impedance line terminals 19, 20 to provide an impedance ratio that is dependent upon the amount of taper between the individual conductors 11 and 12 of line 10. In one particular example, this impedance ratio is 3:1 so that when the desired impedance between the low-impedance line terminals 17 and 13 is 200 ohms, the impedance of the line that is connected to terminals 19 and 20 must be 600 ohms in order to minimize standing waves on the line 10.

The tapered line 10 and the coaxial matching section 13 are contained within box 21 that has walls of conductive material. In order to obtain access to the impedance matching sections, the box is fitted with a conductive top cover 23. Aluminum is a preferred material because it is light weight and quite highly conductive. The box is divided into two compartments by conductive plate 22 that is placed across the box between the tapered line 10 and the coaxial matching section 13. Insulating supports 24 are mounted in two tapered rows on opposite sides of the box 21 by retaining screws 25. The overall taper of the line 10 may be observed most easily by noting the pattern of the screws 25 on the side of the box 21. The top tapered row supports conductor 12 of line 11) and the bottom row supports conductor 11. As viewed from the top, the insulating supports for each of the conductors 11 and 12 are substantially equally spaced. The insulating supports are arranged in a similar pattern on each side of the box so that for each support on one side of the box there is a corresponding support on the opposite side of the box.

With reference to Figure 2, conductor 12 of line 10 extends from low-impedance line terminal 18 to insulating support 24a on one side of the box and is folded back to extend laterally to the opposite 'msulating support 24b and is again folded to extend to insulating support 240 that is adjacent to insulating support 2412. In this manner the conductor 12 is folded and mounted alternately on opposite insulating supports to define a continuous electrically series circuit of physically parallel and approximately equally spaced conductive portions that extend between low-impedance line terminal 18 and high-impedance line terminal 21?. Likewise, conductor 11 extends between low-impedance line terminal 17 and high-intpedance line terminal 19 to define parallel portions, each portion being directly opposite a corresponding parallel portion of conductor 12. The two lines are symmetric-ally positioned relative to a longitudinal axis of the box so that the cap-acitances between corresponding portions of the two conductors and the sides of the box are equal. When the conductor is copper tubing, it can be mounted conveniently to the insulating supports by flattening a short length of the tubing at each insulating support and forming two approximate right-angle bends so that the length of the flat portion of the bends is about one half the distance between the centers of the adjacent supports. A mounting hole in the center of each fiat portion permits the lines to be mounted to insulating supports 24 by screws 26 and nuts 27.

In order to contain line 10 in a desirably small space, the size of both conductors may be changed at required intervals and the tapered configuration of the mounting supports adjusted to maintain a gradual change in the characteristic impedance of the line. The closest spacing in a line that is designed to handle high power is determined by that required to prevent arcing between the individual conductors. In the particular example described herein that has low-impedance line terminals for a 50- ohm unbalanced line and high-impedance line terminals for a 600-ohm balanced line, the power to be transmitted is 50 kilowatts within a range of frequencies from 4 megacycles to 30 megacycles, and the minimum spacing between the conductors that are connected to the lowimpedance terminals is determined by this requirement and the highest expected standing wave ratio.

An exponential taper of the line is obtained from approximate calculations and the final spacing of different size conductors for optimum operation is determined from test results as recorded for different spacings and for different size conductors. The approximate spacing is derived from the well known formula which expresses the relation between the characteristic impedance Z of a two-conductor line and the ratio of the distance .9 between the conductors to the diameter d of each of the conductors. The exact computation of the required spacing between the conductors of a folded exponential transmission line that is contained within a box that has conductive sides relatively close to the line is very complex. The computation would necessarily include capacitance and inductance effects due to the proximity of the different parallel portions of the conductors and the proximity of the conductors to the conductive sides of the box. Fortunately, in practice it is found that these complex relations apparently largely cancel and that the ratio 20 =27e log for the individual parallel portions of the line within the box would not differ greatly from the ratio for open line. For example, the difference between the calculated ratio and the actual ratio may be only about 15%. From test results, the proper spacing between individual opposite portions of the line is determined for obtaining that impedance which is shown by calculation to be required at the various distances along the line to provide an exponential taper.

In the system that is designed for 50 kilowatts, each conductor consists of three different sizes of copper tubing,- having-diameters of /s inch, 4 inch and inch, connected in series. At the end of the line that is nearest the low-impedance line terminals 17 and 18, %-inch tubing is properly spaced by those insulating supports that are in the first group of terminals which are observed in Figure 1 to be in a gradual tapered configuration. The i-inch tubing is supported by the central group of tapered terminals and the As-inch tubing by that tapered group nearest the high-impedance line terminals 19 and 20. The entire line which is actually 84 feet long and the coupling coaxial matching section 13 are contained within a box that measures 75 inches by 37 inches by 16 inches. The length of that portion of the box which contains the folded transmission line is only about 5% of the total length of the line.

Over a limited range of high frequency signals, a 4:1 impedance transformation may be obtained by the use of the well known coaxial type transformer in which the inner conductor of the unbalanced coaxialline divides into two inner conducting sections that are connected individually to terminals for a balanced line. The length of one of the coaxial line sections between the junction of the two lines and its respective balanced terminal is one half wave-length longer than the length of the other line section so that the signal voltages at the balanced terminals differ in phase by 180.

The coaxial matching section 13 has two lengths of coaxial transmission lines 14 and 15 arranged to provide satisfactory matching over a wide frequency range. Unlike the coaxial transformer described above, the 180 transition in phase that is required for obtaining a 4:1

, ratio in impedance is provided entirely by an inductive or transformer efiect between the inner and the outer conductors of coaxial section 15. In addition to this change in phase caused by induction, a change in phase that varies with frequency results from the transit time of the transmission through the lines section 15. An equal transit time for compensation is provided by coaxial section 14 which functions only as a delay line to maintain the required phase relation between the unbalanced line connector 16 and the balanced line terminals 17, 18.

As best shown in Figure 4, the inner terminal of con nector 16 is connected to the inner conductor of section 14 and to the outerconductor of section 15 so that the ungrounded conductor of any external line that is connected to connector 16 is connected through the inner conductor of section 14 to the unbalanced line terminal 17 and through the outer conductor of section 15 to ground. The same end of section 15 that has the connection between the outer conductor and connector 16, has a connection between the inner conductor and ground, and the opposite end of the inner conductor is connected to the balanced line terminal 18. As shown in Figure 4, in order to increase the inductance of section 15 Without increasing the length of the line, the-section is assembled into a coil having insulated turns. The coiled section 15 is mounted to the case by insulating support 29, but the outer conductor of section 14 is thoroughly grounded to the sides of box 21 by several grounding clamps 28 that are spaced along its length.

In the example which is designed for transmitting 50 kilowatts of signal from an unbalanced line to a balanced line, about 10 to 11 feet of 3% inch coaxial transmission line is used for each of the sections 14 and 15. The lines are the same length so that delay time is equal. As shown in Figure 5 the outer conductor of section 15 is connected in parallel with the unbalanced line connector 16 and must therefore have sufiicient length to provide the necessary inductive reactance for preventing a low reactance circuit across the line at the low-frequency end of the operating range. By coiling the coaxial reactor 15 obviously greater inductance and a more compact arrangement is obtained for the total length that is used. If section 15 were a straight line, the maximum permissible length must be substantially less than one half wavelength of the highest frequency that is to be transmitted in order to prevent the ground connection at the end of the section being reflected as a short circuit across the conductors of connector 16. Because of the increase in inductance that is caused by making section 15 into a coil, the permissible maximum length is less than that for a straight section line.

The standing ratio of this impedance matching transmission line is less than 2:1 over its frequency range of 4 megacycles to 30 megacycles. The range is extended through using the folded tapered transmission line in combination with a coaxial impedance matching section 13 for the inductive reactance of the coaxial section is complemented by the capacitive reactance of the folded tapered line section.

Although the matching impedance transformer of this invention has been described with reference to a single embodiment, it is to be understood that the transformer can be modified in ways obvious to those skilled in the art and still be Within the spirit and scope of the following claim.

I claim:

A folded tapered two-conductor line assembly for impedance matching in communication equipments operating at high power over a wide range of frequencies comprising, an electrically conductive box for enclosing said conductors, a plurality of insulators arranged in two similarly inclined and oriented rows on each of two opposite sides of said box, said insulators being mounted to extend inwardly from the sides of said box to support said conductors such as to provide circuitous high-impedance leakage paths between said conductors, each of said conductors extending progressively back and forth between respective ones of said opposite rows so that adjacent transverse portions of the two conductors gradually change in spacing as the line progresses from one end of the box to the other to form said folded tapered line, and said line being symmetrically placed relative to the 20 page 908.

sides of said box to equalize the capacitance between the individual conductors of said line and the sides of the box.

References Cited in the file of this patent UNITED STATES PATENTS 2,018,320 Roberts Oct. 22, 1935 2,204,712 Wheeler June 18, 1940 2,231,152 Buschbeck Feb. 11, 1941 2,391,880 Chesus Jan. 1, 1946 2,462,410 Lindenblad Feb. 22, 1949 2,777,996 Bruene Jan. 15, 1957 FOREIGN PATENTS 1,006,915 Germany Apr. 25, 1957 OTHER REFERENCES Electronics, Engineering Edition, Aug. 15, 1958, page 102. (Copy available in Scientific Library.)

Primozich et 21.: Electrical Engineering, October 1955,

(Copy available in Scientific Library.) 

